1. Field of the Invention
The present invention relates to high chromium steel for use as a high temperature material, consisting of a mixed structure containing ferrite and which possesses excellent oxidation resistance and high temperature strength, good weldability and toughness, and is thus suited for use as heat exchanger material in chemical industry equipment and machinery.
2. Description of the Prior Art
Materials that serve to transmit heat inside and outside the pipe as the super heater piping and reheater piping of the boiler, the heating piping and heat exchanger piping of equipment and apparatus for the chemical industry require sufficient oxidation resistance and high temperature strength to satisfy the conditions under which they are used as well as good weldability since almost all of them are assembled by welding. Also desirable is an economical cost.
As an example, the steel presently used in the superheater pipe and reheater pipe of a power plant boiler is 21/4Cr-1 Mo steel for pipe wall temperatures up to 590.degree. C., 9 Cr-1 Mo steel or 18 Cr-8 Ni stainless steels for temperatures up to 650.degree. C., and 18 Cr-8 Ni stainless steel for temperatures above 650.degree. C.; the higher the service temperature, the better the quality of the material used. With 21/4Cr-1 Mo steel, there is no difficulty with respect to the oxidation resistance and high temperature strength under the normal working conditions as well as in the toughness and economical requirements. In the case of 9 Cr-1 Mo steel, the oxidation resistance is sufficient when the pipe wall temperature is below 650.degree. C., and there is no problem with the toughness. However, the disadvantage is that the high temperature strength is insufficent and the scope of application may be limited depending on the working stress. The most serious defect of this steel is its poor weldability, and it is often liable to incur welding cracks. 18 Cr-8 Ni stainless steels retain sufficient oxidation resistance and high temperature strength even when used at 650.degree. C. or higher temperatures and are in extensive use and applications. They are, however, unfavorable in that the material cost is higher because of high nickel content than in case of a 9 Cr-1 Mo steel.
A grade of high chromium steel having a single ferrite phase has been proposed for the purpose of eliminating the aforesaid defects encountered in conventionally used steels. A sufficiently high strength at high temperatures may be provided by properly adjusting the strengthening elements and there is little possibility of hardening because of its single ferritic phase; consequently, there is no problem with the weldability. One problem of this steel, however, is that the toughness is not satisfactory. In other words, its impact fracture transition temperature is fairly high. For this reason, it is inevitable that the application of this steel is restricted. Also proposed is a high chromium steel of a two-phase structure with the object of eliminating the above-mentioned defects of the single ferrite phase steel. However, with this steel, a ferrite forming element, such as, titanium and aluminum, is added to obtain a two-phase structure and a carbide forming element, such as, vanadium and niobium is also added to secure high temperature strength. However, special consideration must be paid to the prevention of the oxidation loss and other factors for adjusting the composition since highly oxidizing titanium and aluminum are present, which can also present problems from an economical viewpoint.